This invention relates generally to a process and apparatus for the removal of organic sulfur compounds from a stream containing hydrocarbons. In particular, it relates to the use of an aqueous caustic solution to remove mercaptans from a hydrocarbon stream.
It is conventional practice at the present time to treat sour hydrocarbon and gas streams to remove mercaptans. Extraction processes are typically used when treating light hydrocarbons and gas streams for mercaptan removal. Mercaptans have traditionally been removed from, hydrocarbon streams because of their malodorous scent.
U.S. Pat. No. 5,244,643 B1 discloses a process whereby a hydrocarbonaceous gas stream including mercaptan sulfur, air and aqueous alkaline solution including a mercaptan oxidation catalyst are mixed in a mixing vessel in which mercaptans are converted to disulfides. The effluent withdrawn from the top of the mixing vessel is settled in a vessel to yield separated streams of air, liquid hydrocarbon product containing disulfide and an aqueous alkaline solution including mercaptan oxidation catalyst.
U.S. Pat. No. 4,562,300 B1 discloses contacting a hydrocarbon stream including organic mercaptans with sodium hydroxide to free the hydrocarbons from the organic mercaptans. The caustic solution rich in mercaptans is oxidized with a catalyst and the organic mercaptaris are converted to organic disulfides. The mixture of regenerated caustic solution free of organic mercaptans and organic disulfides enter a settler in which the organic disulfides and caustic solution are separated. A stream of hydrocarbon from which the mercaptans had been extracted upstream are admixed with regenerated aqueous caustic solution containing small amounts of organic disulfides to extract the organic disulfides from the regenerated caustic solution. These processes leave disulfides in the liquid hydrocarbon stream. Anticipated and current governmental regulations are increasing the undesirability of leaving organic disulfides in a liquid hydrocarbon product stream.
In a liquid-liquid extraction process, disulfides are removed from the hydrocarbon stream not to be returned. A liquid-liquid extraction process is generally described in J. R. Salazar, Handbook of Petroleum Refining Processes 9-4-9-5 (Robert A. Meyers ed. 1986). In a typical extraction process, a liquid hydrocarbon stream is fed to an amine absorber column to be contacted with an amine, such as diethylamine, to absorb acid gases such as hydrogen sulfide and carbon dioxide from the hydrocarbon stream. The hydrocarbon stream lean of hydrogen sulfide and other acid gases is prewashed in a prewash vessel containing 6.5 to 7.0 wt-% liquid caustic to convert the remaining hydrogen sulfide to sodium sulfide which is soluble in caustic. The hydrocarbon stream, now depleted of hydrogen sulfide, is subjected to counter-current flow of about 14 wt-% liquid caustic in an extractor vessel. Jet decks in the extractor vessel facilitate the counter-current contact. Mercaptans in the hydrocarbon stream react with the caustic to yield mercaptides. The mercaptides in the hydrocarbon stream are soluble in the caustic. A product hydrocarbon stream lean in mercaptans passes overhead from the extraction column through a settler drum to a sand filter vessel, and the mercaptide rich caustic passes from the bottom of the column. The settler drum allows for settling and buffers the sand filter against caustic surges. The sand filter coalesces caustic to make it gravitate to the bottom of the vessel while the product hydrocarbon stream passes out of the vessel through an outlet that is shielded at the top to prevent admittance of falling caustic droplets. The mercaptide rich caustic receives an injection of air and catalyst as it passes from the extraction column to an oxidation vessel for regeneration. Oxidizing the mercaptides to disulfides using a phthalocyanine catalyst regenerates the caustic solution. A disulfide separator receives the disulfide rich caustic from the oxidation vessel. The disulfide separator vents excess air and decants disulfides from the caustic before the regenerated caustic is drained and returned to the extractor vessel. The disulfides are run through a sand filter and removed from the process.
Hydrogen sulfide must be removed in the prewash vessel before extraction or the caustic will preferably react with the hydrogen sulfide in the extractor vessel and leave mercaptans in the hydrocarbon stream. Eventually, caustic in the prewash vessel becomes overloaded with sulfides and must be replaced to ensure adequate conversion of hydrogen sulfide. In the above-described mercaptan extraction system, the caustic in the prewash vessel had to be replaced on a batch basis. Consequently, conversion of hydrogen sulfide at the late end of the replacement cycle is lower. The prewash vessel also has to be large to assure adequate mixing of the caustic and the hydrocarbon. Moreover, the conventional mercaptan extraction system utilizes four vessels: the caustic prewash vessel, the extraction vessel, the settler drum and the sand filter vessel. Each vessel must have its own support and base structure making building this system capital-intensive.
Accordingly, it is an object of the present invention to combine the extractor vessel, the settler drum and the sand filter vessel into one vessel to thereby reduce the capital required to build an extraction system. Moreover, it is an object of the present invention to combine the caustic prewash vessel, the extractor vessel, the settler drum and the sand filter vessel into one vessel to thereby reduce the capital required to build an extraction system.
We have developed a process and apparatus for combining the functions of the extractor vessel and the sand filter vessel into one common extractor vessel. A coalescer is disposed at the top of the extractor section of the vessel to serve the function that the sand filter previously provided. The coalescer may be spaced apart from a topmost jet deck in the extractor section by sufficient volume to buffer against caustic surges to serve the function of a settler drum.
We have also developed a process and apparatus for combining the functions of the prewash vessel, the extractor vessel, the settler drum and the sand filter vessel into one common extractor vessel. An extractor section is disposed directly above a caustic prewash section.
Accordingly, in one embodiment, the present invention relates to an apparatus for converting sulfur compounds in a hydrocarbon stream. The apparatus comprises a prewash section for converting hydrogen sulfide to sodium sulfide. A hydrocarbon feed conduit has an inlet in communication with the prewash section. An extractor section for converting mercaptans to mercaptides is disposed directly above the prewash section. A hydrocarbon product conduit has an outlet in communication with the extractor section. A conduit has an outlet in communication with the prewash section and an inlet in communication with the extractor section.
In another embodiment, the present invention relates to a process for converting sulfur compounds in a hydrocarbon stream. A hydrocarbon stream containing sulfur compounds is fed to a prewash section containing alkali to convert hydrogen sulfide to sodium sulfide. A prewashed hydrocarbon stream is withdrawn from the prewash section. The prewashed hydrocarbon stream is fed to an extractor section to convert mercaptans to mercaptides. The extractor section is directly above the prewash section. An extracted hydrocarbon stream containing mercaptides is withdrawn from the extractor section.
In a further embodiment, the present invention relates to an apparatus for converting sulfur compounds in a hydrocarbon stream. The apparatus comprises an extractor section for converting mercaptans to mercaptides. A hydrocarbon feed conduit has an outlet in communication with the extractor section proximate a bottom of the extractor section. A hydrocarbon product conduit has an inlet in communication with the extractor section proximate a top of the extractor section. A structure for facilitating contact between alkali and hydrocarbon is disposed between the outlet of the hydrocarbon feed conduit and the inlet of the hydrocarbon product inlet. An alkaline conduit has an inlet disposed below the outlet to the hydrocarbon feed conduit. A coalescer in the extractor section is disposed between the inlet to the hydrocarbon product conduit and a top of the structure for facilitating contact between alkali and hydrocarbon. The coalescer permits no more than 1 ppm of alkali to pass therethrough.
Additional objects, embodiments and details of this invention can be obtained from the following detailed description of the invention.